1. Field of the Invention
The present invention relates to sensors for detecting celestial bodies radiating infrared energy and, more particularly, to electronic circuits utilized in sensors for detecting celestial bodies radiating infrared energy.
2. Description of the Related Art
Spacecraft often use celestial bodies such as the earth or the sun as reference objects. Data regarding the position or attitude of the spacecraft relative to such bodies is used to monitor the spacecraft's progress and to make necessary corrections in its position or attitude. Thus, spacecraft frequently are equipped with sensors for detecting such celestial bodies.
One type of sensor, for example, is an infrared horizon sensor which senses the crossing of a horizon of a celestial body as the spacecraft moves relative to the body. The horizon sensor converts infrared energy radiating from the body into electronic signals which are processed to provide desired information. For example, the time delay between the crossings of the two horizons of a particular celestial body may be compared with a reference time delay, and a resultant electronic error signal may be generated. The error signal then may be used for making appropriate corrections in the spacecraft's attitude relative to the celestial body.
Earlier infrared horizon sensors typically were equipped with an electronic amplifer circuit for amplifying electronic signals generated in response to celestial bodies. Often, an infrared horizon sensor was configured to sense more than one celestial body on a periodic basis, and frequently the intensity of the infrared energy radiating from one periodically sensed celestial body, such as the sun, was significantly greater than that radiating from another periodically sensed celestial body, such as the earth. As a result, the magnitude of the electronic signals generated in response to the celestial body emitting the higher intensity infrared energy often was significantly greater than the magnitude of the electronic signals generated in response to the celestial body emitting the lower intensity infrared energy.
Unfortunately, the magnitude of the electronic signals responsive to the higher intensity body often was so great that it could drive an unprotected electronic amplifier circuit into a saturation condition in which the horizon sensor was temporarily blinded and inoperable. Furthermore, the period of time necessary for the electronic amplifier circuit to recover from the saturation condition could be long enough to mask one or more subsequent sweeps of a reference celestial body, and valuable data regarding the attitude of the spacecraft could be lost.
In order to protect against saturation of an amplifier circuit, earlier infrared horizon sensors often included a limiting circuit for limiting magnitude of the electronic signals provided to the amplifier circuit. While such earlier limiting circuits, generally were successful, there were shortcomings with their use. More particularly, the earlier limiting circuits often were unstable and experienced unwanted loop oscillations due to their inability to satisfactorily meet stability criteria such as gain and phase margins.
Thus, there has been a need for an improved circuit, for use in a sensor for detecting infrared energy radiating from celestial bodies, which is stable and substantially free from unwanted loop oscillations. The present invention meets this need.